Bacterial pneumonia is a frequent and deadly complication in hospitalized patients. Pseudomonas aeruginosa (PA) is an aerobic gram-negative bacterium that is the second most common cause of pneumonia in hospitalized patients, with mortality ranging from 60-90% in mechanically ventilated with pneumonia due to PA pneumonia. Innate host responses to PA are initiated by selected toll like receptors (TLR), including TLR4, which recognizes LPS, and TLR5, which is activated by flagellin. Flagellin stimulates protective immunity against diverse microbial pathogens, including bacteria and viruses. We have recently found that flagellin is a major inducer of the cationic peptide cathelicidin related antimicrobial peptide (CRAMP). CRAMP exerts important bactericidal and immunomodulatory properties that may contribute to the protective effects of flagellin observed in both infectious and non-infectious models. The central hypothesis of this revised proposal is that Pseudomonas flagellin stimulates protective lung innate mucosal immunity, which is mediated, in part, by the induction of the cationic antimicrobial peptide CRAMP. The Specific Aims of the proposal are as follows: Specific Aim 1. Determine the role of the cationic antimicrobial peptide CRAMP in mucosal innate responses during PA pneumonia Specific Aim 2. Determine the mechanism of flagellin-induced stimulation of protective lung innate antibacterial immunity The studies proposed will provide important insights into the role of cathelicidins in lung antibacterial host defense, and may potentially identify novel approaches to stimulate lung mucosal immunity that can be used to prevent or treat pneumonia in hospitalized patients.